The present invention relates to an air/fuel ratio detecting arrangement for detecting the mixing ratio of intake air to fuel or air/fuel (A/F) ratio of an automotive engine, for example.
Generally, automotive engines provide an A/F ratio sensor arranged with an exhaust pipe for sensing the content of oxygen remaining in exhaust gas as A/F ratio of intake air. In order to obtain the A/F ratio close to a theoretical A/F ratio (=14.7) during engine idle operation, for example, an electronic control unit (ECU) carries out correction control of the fuel injection amount, i.e. so-called A/F ratio control.
The A/F ratio sensor arranged with the exhaust pipe constitutes an A/F ratio detecting arrangement as disclosed, for example, in U.S. Pat. No. 5,236,569 issued Aug. 17, 1993 to Murase et al. And the A/F ratio sensor includes a casing formed out of an oxygen-ion conductive material with such as zirconia (ZrO2) and having air and gas cells.
Arranged in the sensor casing are an oxygen sensor part for providing an oxygen content signal obtained in accordance with the oxygen contents in the air and gas cells and by means of a first electrode disposed on the air-cell side and a second electrode disposed on the gas-cell side, an oxygen pump part for urging oxygen ions to flow into or out of the gas cell through the sensor casing by providing an oxygen pump signal to a third electrode disposed on the gas-cell side and a fourth electrode disposed outside, and a heater for heating the oxygen sensor part and the oxygen pump part by receiving outside electric power.
The A/F ratio sensor is connected to a signal output processing circuit or A/F ratio outputting means. The signal output processing circuit provides an oxygen pump signal to the third and fourth electrodes in accordance with an oxygen content signal derived from the first and second electrodes, and it also provides to the ECU, etc. an A/F ratio signal indicative of the oxygen content in the gas cell in accordance with the electric power amount supplied to the third and fourth electrodes by the oxygen pump signal.
During engine operation and when exhaust gas flows into the gas cell of the sensor casing, oxygen ions are urged to move through the oxygen sensor part in accordance with a difference in oxygen content between the air and gas cells, generating an electromotive force or an oxygen content signal between the first and second electrodes.
The signal output processing circuit applies a voltage signal or oxygen pump signal between the third and fourth electrodes in accordance with an oxygen content signal generated between the first and second electrodes so as to urge to move oxygen ions through the oxygen pump part in such a way as to compensate a difference in oxygen content between the air and gas cells. As a result, current is passed between the third and fourth electrodes in accordance with the amount of moved oxygen ions. Using a value of this current or pump current, a voltage value of an oxygen pump signal, etc., the signal output processing circuit provides an A/F ratio signal indicative of the A/F ratio of intake air.
Through the A/F ratio signal, the ECU can receive the A/F ratio of intake air as a continuous value, based on which accurate A/F ratio control can be ensured to obtain the A/F ratio close to the theoretical A/F ratio.
In the above A/F ratio detecting arrangements, upon engine start, etc., the oxygen sensor part and the oxygen pump part are heated by the heater for their quick activation. Upon engine start, for example, the two parts are heated up to about 550.degree. C. during at least 18-20 sec. so as to make the A/F ratio detectable.
In the A/F ratio detecting arrangements, as described above, upon engine start, for example, the oxygen sensor part and the oxygen pump part are heated by the heater up to about 550.degree. C. for their quick activation. This means that upon engine start, a time of at least 18-20 sec. is needed from start of heating to full activation of the two parts. Additionally, when the oxygen pump part is not activated yet, the signal output processing circuit provides an A/F ratio signal having roughly the same output value as that when the A/F ratio is close to the theoretical A/F ratio. It is thus difficult to early determine whether or not the oxygen pump part is activated by the simple use of the A/F ratio signal.
Therefore, in the above A/F ratio detecting arrangements, detection of the A/F ratio is often started after a wait of about 18-20 sec., during which the oxygen pump part seems to fully be activated. This disallows early commencement of A/F ratio control during idle operation upon engine start, etc., resulting in tendency to poor purification of exhaust gas, etc.